1. Field of the Invention
The invention relates to a process, apparatus and heat microscope for testing the properties of materials in accordance with the photothermal effect.
The principle of photothermal testing or measuring processes is based on irradiating a test surface with light, in particular laser light, and evaluating the heat signals generated thereby in the layers closest to the surface. Such processes exploit the fact that a body heated relative to its surroundings always tends to dissipate its extra heat. The body emits heat in the form of infrared radiation. In principle, the process is usable even if the temperature of the surroundings is higher than that of the specimen, because the decisive factor is the temperature distribution at the surface of the specimen. By measuring the infrared (IR) light signals emitted by the specimen, depth information and information on the material properties of the surface can be obtained. Changes in the layer thicknesses of the surfaces, as well as cracks, inclusions and delaminations, for instance, can be ascertained. Naturally, all of the testing is performed in a non-destructive and contactless manner. The invention is a departure from the basically known process steps of irradiating the surface of the material sample with an intensive light source, particularly a laser. In a number of photothermal test processes the beam is modulated. In other words, it is periodically interrupted, in particular. The laser light is partly converted into heat at the surface. The heat penetrates the sample of material. One characteristic for the measurement signal formed of emitted IR light signals is how far the heat penetrates. Such a characteristic depends firstly on the periodic duration of the irradiation, which is determined by the modulation frequency, and secondly on the material properties of thermal conductivity, specific heat and density. The last three parameters are combined into a physical variable known as the thermal diffusion length .mu..sub.S. The variable directly indicates the penetration depth of the heat waves. The equation is EQU .mu..sub.S =.sqroot.(2a/.omega.),
where
.omega.=radian frequency of the modulation of the intensity-modulated laser beam; PA1 a=temperature conductivity, wherein the following equation applies for a: EQU a=k/.rho..times.c, PA1 c=specific heat, PA1 .rho.=density and PA1 k=thermal conductivity of the specimen.
where
2. Description of the Related Art
Published European Application No. Al 0 105 078 discloses an apparatus for performing the above-described process for testing the properties of absorptive materials by the photothermal effect, in which laser waves from a stationary or quasi-stationary laser can be carried by means of flexible fiber optic cables to a test head that can be optically coupled to the sample of material. The IR light signals can also be transmitted from the test head to an infrared detector located at a distance from the test head, again through flexible fiber-optic cables. The measuring head itself contains beam-carrying means inside its housing, which are located at the laser beam entry and at the IR light signal exit and are in the form of one focusing lens each, as well as a coupling mirror disposed in the beam path of the two lenses and constructed as a dichroic beam splitter. A dichroic mirror or dichroic beam splitter acts as a beam splitter which reflects one wavelength range of radiation or light falling on it and admits a different wavelength range. The uses of such dichroic mirrors include use thereof as color splitters for color television transmission. A crystal rod, particularly of sapphire, is also disposed in the common light path of the laser beams and the IR light signals. The rod is followed by a focusing lens that focuses the laser beam onto the material sample or receives the IR light signals from it. The fiber optical wave guides are a vulnerable element in materials testing and they damp the laser beam or IR light signals carried through them.